MathGroup Archive 2007

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Finding position of an element in a list:


Hi, Gopinath, 

    The problem is that dropOuterBraces is turning the list of values returned 
by Nearest into strings. Then Position isn't able to determine where these 
strings appear in the original list -- because they don't!

     My recommendation is something like the following:
         * Use Nearest to grab the element(s) closest to what you want;
         * Use Position directly on those elements (not their string 
representations;
         * Flatten the result (if your original list is one-dimensional). 

    It's basically exactly what you did, except for the step of turning things 
into strings.

    Here's a one-liner to do this:

In[27]:= Flatten@(Position[xlis, #] & /@ Nearest[xlis, 6])

Out[27]= {7}

    Hope that helps!

         C.O. 




On Thursday 06 December 2007 00:43:15 Gopinath Venkatesan wrote:
> Hello Friends,
>
> (This is an easy problem, but I was unable to get it work in Mathematica,
> probably because of the precision - I tried setting precision too, still
> got stuck)
>
> Suppose we have a list, xlis, having some elements, and I have to find one
> nearest value I am interested in that list. In the below example, I wanted
> to find the position of a number belonging to list and close to 6 (which is
> 5.5 located at 7th).
>
> I can do this:
>
> xlis = {0, 1, 3, 10, 4, 5, 5.5, 10.25};
> dropOuterBraces[lst_List] :=
>  StringTake[#1, {2, StringLength[#1] - 1}] &[
>   ToString[lst]]; (* took from previous help posted by Peter Pein *)
> n1 = Nearest[xlis, 6]
> eval = dropOuterBraces[n1]
> Position[xlis, eval]
> Position[xlis, 5]
>
> As you see, the last but one command returns nothing, and we have to
> explicitly say the number to return the position.
>
> Or should some other command be used to extract the position of an
> arbitrary number in a list?
>
> Same with the case of MemberQ command, it gives True for some and False for
> some. For example, consider this list,
>
> xlis = Table[N[((1 - Cos[(j - 1)/9 \[Pi]])/2) 1.32], {j, 1, 10}];
> dropOuterBraces[lst_List] :=
>  StringTake[#1, {2, StringLength[#1] - 1}] &[ToString[lst]]
>
> eval = Nearest[xlis, 0.32];
> xlis
> elem = dropOuterBraces[eval]
> MemberQ[xlis, 0.545392]
> MemberQ[xlis, 0.33]
> Position[xlis, 0.33]
> Position[xlis, elem]
>
> Please hint where I am wrong.
>
> Thanks,
>
> Gopinath Venkatesan
> Graduate Student
> University of Oklahoma



-- 
==========================================================
Curtis Osterhoudt
cfo at remove_this.lanl.and_this.gov
PGP Key ID: 0x4DCA2A10
Please avoid sending me Word or PowerPoint attachments
See http://www.gnu.org/philosophy/no-word-attachments.html
==========================================================


  • Prev by Date: Re: Adding coloured Polygon to a BarChart
  • Next by Date: Re: Eigensystem consistency
  • Previous by thread: Finding position of an element in a list:
  • Next by thread: Re: Finding position of an element in a list: